1. Field of the Invention
This invention relates to improvements in an air-fuel ratio control system for an internal combustion engine of an automotive vehicle or the like, arranged to accomplish a feedback control of the air-fuel ratio of an air-fuel mixture to be supplied to the engine in accordance with a detection signal representative of the air-fuel (oxygen-combustibles) ratio in exhaust gas from the engine.
2. Description of the Prior Art
Most automotive vehicles are equipped with a so-called three-way catalytic converter for simultaneously converting harmful three components CO, HC (hydrocarbons), NOx (nitrogen oxides) in exhaust gas from an internal combustion engine. In such automotive vehicles, the air-fuel ratio of an air-fuel mixture is feedback-controlled to be fall into a narrow range including stoichiometric value as a center, in response to an air-fuel (oxygen-combustibles) ratio in exhaust gas which ratio is detected by an oxygen sensor located in an exhaust pipe. The three-way catalytic converter effectively works only within the narrow range of the air-fuel ratio of the mixture to be supplied to the engine.
In the feedback-control, an actual fuel supply amount to the engine is determined by correcting a basic fuel supply amount depending upon some engine operating parameters, mainly with a so-called air-fuel ratio feedback correction coefficient (amount) .alpha.. The air-fuel ratio feedback correction coefficient is renewed upon being corrected with a so-called step amount and a so-called integrated amount. Immediately after the air-fuel ratio is inverted from its lean side to its rich side, the step amount is applied to the air-fuel ratio feedback control coefficient in order to return the air-fuel ratio to the lean side at a high response, and thereafter the integrated amount of a smaller value is applied to the air-fuel ratio feedback control coefficient until the air-fuel ratio is inverted to the rich side, thus stabilizing the feedback control. When the air-fuel ratio is inverted from the rich side to the lean side, a control is made in a reverse manner relative to the above.
Even under the above feedback control, the air-fuel ratio of the air-fuel mixture to be supplied to the engine unavoidably largely shifts from the lean to the rich side or vice versa in the event that the air-fuel ratio is disturbed by acceleration, deceleration and outside disturbances such as fuel purge from a carbon canister, gear shift and EGR (exhaust gas recirculation). If the thus largely shifted air-fuel ratio is intended to be corrected by the integrated amount which is set corresponding to a steady state (engine operating conditions other than the acceleration, deceleration and the outside disturbances), the correction control with the integrated amount follows or continues for long. Accordingly, in the even that the air-fuel ratio largely shifts to the rich side, HC and CO are unavoidably emitted from the catalytic converter in the course of returning the air-fuel ratio to the lean side. On the contrary, in the event that the air-fuel ratio is shifts largely to the lean side, NOx is unavoidably emitted in the course of returning of the air-fuel ratio to the rich side.
In order to solve the above problems, a proposition has been made as disclosed in Japanese Patent Provisional Publication No. 58-106150, which is arranged as follows: A so-called lapsed engine revolution number is measured since the air-fuel ratio is inverted from the lean side to rich side or vice versa. The thus measured lapsed engine revolution number is compared with a decision standard. When the measured lapsed engine revolution number exceeds the decision standard, the value of the integrated amount is increased. As shown in FIG. 23, a smaller integrated amount I1 is applied for a while from a time immediately after the inversion of the air-fuel ratio. However, if the air-fuel ratio is not inverted even after a while and the lapsed engine revolution number is brought into agreement with the decision standard, an integrated amount 12 having a large inclination is applied from that time point. Thus, the value of the integrated amount is increased in the course of an air-fuel ratio feedback control cycle, thereby following the air-fuel ratio largely shifted to the rich side or the lean side at a high response.
However, drawbacks have been encountered even in the above proposition, as set forth below. In the above proposition, the decision standard C1 for changing the magnitude of the integrated amount has been previously set, in which, for example, the value of the integrated amount is increased if a predetermined time has lapsed from the time at which application of the integrated amount is initiated. Accordingly, there arises a problem that correction by the integrated amount becomes excessive or deficient, owing to difference among engines, difference in response performance among oxygen sensors, or deterioration in response performance of the oxygen sensor with age.
This will be discussed more specifically with reference to FIG. 24 which shows the change in control frequency of an oxygen sensor in terms of a distance traveled by an automotive vehicle. As shown in FIG. 24, the control frequency at the time of initiation in use is about 2.0 to 2.5 Hz when the oxygen sensor is new, and it lowers as the vehicle travel distance increases. The control frequency lowers to the lowest value of about 1.4 Hz. Additionally, it is observed that the distribution of the control frequency relative to the average value is enlarged as the vehicle traveled distance increases.
If the control frequency of the oxygen sensor thus changes, the control cycle of the air-fuel ratio feedback correction coefficient .alpha. is changed. This is because the control cycle of the air-fuel ratio feedback correction coefficient .alpha. depends on the output of the oxygen sensor. Thus, when the reaction of the oxygen sensor is retarded under a deteriorated response performance, the control cycle of the air-fuel ratio feedback correction coefficient .alpha. is prolonged. Additionally, the control cycle of the same coefficient is changed in accordance with difference in response performance among oxygen sensors.
Assume that the decision standard C1 is determined so as to increase the integrated amount when the lapsed engine revolution number becomes 3 from the time of the inversion of the air-fuel ratio between its lean and rich sides, so that the decision standard is in agreement with a new oxygen sensor. In this case, if the control frequency of the air-fuel ratio feedback correction coefficient .alpha. is prolonged owing to deteriorated response performance of the oxygen sensor, the timing of increasing the integrated amount is advanced thereby excessively increasing the amount of feedback correction.